There are numerous control applications in which a reliable electrical signal representative of the speed of a rotating shaft is essential. For example, in a bus or other passenger vehicle it may be desirable to provide an interlock to prevent opening of the doors whenever the vehicle is moving above a given speed. Similarly, in any vehicle it may be desirable to provide an interlock control to prevent shifting the transmission into reverse gear while the vehicle is moving in a forward direction at even a very low speed. There are also numerous industrial applications which require active or preventive control for one part of a machine whenever some other part of the machine is operating either above or below a particular critical speed.
To meet the needs of these various applications, a signal generator that provides a useful signal amplitude over a broad speed range is highly desirable. This presents substantial problems, particularly at relative low speeds. The signal generator must also afford a consistent relationship between some parameter of its signal output and the speed of the rotating member that it monitors. Thus, there should be a consistent relation between either the signal amplitude and the speed of the rotating member of between the signal frequency and the speed of the rotating member. Preferably, for maximum flexibility in various different applications, both the amplitude and the frequency of the output signal from the generator should be consistently related to the speed of the rotating shaft or other member being monitored, particularly at low speeds.
Sub-fractional permanent magnet signal generators have been employed in applications of the kind described above, but present substantial operating difficulties and cost problems. These devices are A.C. generators, and effective utilization of their output signals frequently requires a relatively consistent wave form. However, the wave form of the output signal is often subject to substantial variation due to lack of concentricity in the signal generator components. Furthermore, the same lack of concentricity may produce substantial variations in signal amplitude, at corresponding speeds, from unit to unit. These difficulties can be alleviated by adopting high precision manufacturing techniques and relatively expensive structures, but the resulting cost is frequently prohibitive, particularly for high volume applications in the automotive field.
In many applications, the speed signal generator is most conveniently used by being incorporated in an existing mechanism employed for other purposes. For example, in many automotive applications, where the speed of movement of the vehicle is the critical factor to be monitored, it is most convenient to connect the signal generator to the flexible rotating cable that drives the speedometer. In an application of this kind, the signal generator should provide for rapid and convenient connection to the speedometer cable, either at the transmission end or at the speedometer end, and should allow the speedometer drive cable to extend through the signal generator to maintain its original use.
In other applications, there may be two critical speeds for the same rotating member. Again referring to the automotive field, it may be desirable to provide for locking vehicles doors above one critical speed and to lock out the transmission from a shift into reverse at a substantially lower critical speed. To meet the needs of applications of this kind, the signal generators should be capable of being stacked in series with each other.